titanium coloring - brazing titanium
 

I have a couple of ti stems hanging around that I thought would make good sacrifices to color by electrolysis or heat and wondered if anyone has had any success with this?
You see it all the time with jewelry and I know there was someone in the industry that did one off stuff but can't pull it out of the loose brain cells.
I also got in a discussion with a weldor on the jobsite about brazing ti and thought there was someone up in Canada who once did this. I know ti can be brazed I just remember someone actually doing it. This big dawg welder claimed it can't be done.
I suppose this is two threads in one but thanks for any and all answers.
Any suggestions would be appreciated.

Moroni was the man, tell me where to send your half of the 20 I got off the welder on my job site. Thanks

For information - titanium alloys can be brazed, in a manner. There are basically two jointing alloy options - a low temperature aluminium-magnesium based alloy system range which is a real dissimilar 'brazing' system; and a higher temperature nickel-titanium system range, with compositions all the way between the two parent elements whic is closer to a dissimilar wleding fille rin a lot of cases.

Titanium Brazing Inc, of Ohio do a range of filler metals, powders and foils for the purpose. I'm sure a number of other companies will, but that's justy of the top of my head.

On Q2, as far as I know the answer is as per the previous posters - it can be done but it's not worth the considerable effort involved.

On Q1, the answer is subtly different - it can be done and it's not hard but it's still not worth the effort. The colour is due to the presence of a thin oxide layer which refracts light. The oxide structure has a different atomic spacing from the parent metal and this causes the surface to be placed in tension. The residual tensile stress means that a small crack or scratch is more likely to continue to expand, so the effect is to weaken the material.

Paint, on the other hand, doesn't have this problem.

;)

http://www.titaniumarts.com/, even the dimmest lightbulb still has a glow, Leni Fried Designs is the resource I was scratching to remember. Thanks to all posters, the answers are always appreciated.

No, but paint does have the problem of adhering to titanium in the first place, much like aluminium. You'll need an etch primer.

I'm not sure who told you that a layer of surface oxide on titanium is thick enough to even begin to generate surfaces stresses great enough to be measured with anything more than an AFM. The oxide film on CP titanium and most of its near alpha and alpha-beta alloys is of the order of 1 nanometre. Artifically thickened films, either by heat tinting or anodizing will take you up to about a maximum of 30 nm Even with a lattice mismatch of 200%, which is a ludicrous figure picked solely for this example, you're still experiencing very, very low purely elastic strain at the maximum growth.

Titanium alloys, much like steel alloys, are known for their ability to undergo considerable plastic work at sites of stress intension. They are tough. The slightest surface cracking in an oxide film of that thickness will simply blunt upon penetration of the substrate surface and cease to be considered, the exposed metal repassivating.

In short: Anodising or heat-tinting is in no way compromising the surface of the structure.

Falanx, I'm happy to be proved wrong by someone who actually works in this field. Always willing to learn.

The information regarding possible surface stresses came from my interpretation of several articles on shot peening of Ti alloys, some of which showed measureable reductions in fatigue strength of anodised Ti alloy. See for instance this one, esp fig 2.

I don't understand where your 30 nm figure comes from, I thought the colour effect needed a quarter wavelength to occur, that's a minimal thickness of 80 nm.

I know how hard it is to get anything including paint to stick to Ti. I understood that an agent which modified the surface activity (eg silane) was the best bet, on the grounds that etching Ti wasn't easy.

My apologies. That should have read 300nm. I didn't notice. Whoops! Above that, you're looking at heavily crystallised coating intended for gross corrorion resistance, or wear resistance (although on titanium, any very hard layer is really a bad idea - the substrate will never be hard enough to properly support it). Most oxide layers on Ti are about 3nm per volt and decorative anodizing doesn't go much above 20 v...

Anyways, a third of a micron isn't a sizeable enough percentage of sectional thickness to dominate the mechanical properties. More than two microns on the other hand is, especially in thin sections or rods, and especially when the crystals are coarse as they are in big, thick coats. The oxide films grown on those samples in the paper you've shown are heavy, heavy layers.

As for the surface modification, anodizing itself serves as a good etch. But it needs to be pretty thick. See where I'm going?

OK and thanks.

If I have this right, the take home message is that the kind of decorative anodising we're talking about isn't thick enough to cause appreciable residual surface stress. The question that springs to mind is therefore how durable is this kind of thin coating?

It's irrelevant really. Titanium is plenty chemically resistant with an air-formed film of oxide - one or two nanometres thick, and it's thermodynamically only a little less stable than aluminium oxide - and that forms in the 'vacuum' of space.